When I think of a portamento, I think of a pitch bend from one note to the next, so it's a smooth transition instead of a cut n try note change.
I don't know of any plugins that will necessarily do that easily like you're asking... unless it's included within the plugin.
I supposed you could manufacture it yourself by using a pitch bender plugin. If I were going to do it. I would keep the notes the same and use something like Waves Soundshifter or QuikQuak Pitchwheel, and automate the pitch from 1.0 to about the right note. I would use a Guitar Tuner plugin to make sure I got the right note. That will definitely work.. but at the same time is not quick & easy. That might be as good as you can get without using the plugin's portamento.